Laser photocoagulators have become an important tool in treating eye disease. As stated in an article entitled "Lasers In Ophthalmology: The Path From Theory To Application" by M. L. Wolbarsht and M. B. Landers III in Applied Optics, Vol. 18, No. 10, May 15, 1979, pp. 1516-1526, specifically at p. 1516:
"The argon laser photocoagulator is routinely used by ophthalmologists all over the world and is now standard practice for the treatment of many retinal diseases. Indeed for some problems such as diabetic retinopathy, not to use it borders on malpractice."
Although retinal photocoagulation has been used since before the advent of lasers and is now a standard treatment of choice for some common retinal disorders, the occurrence of complications is high, compared to other kinds of laser therapy. Because of variations in vascularization and pigmentation from place to place on the fundus, obtaining an optimum application of laser energy requires a delicate adjustment of laser energy, laser pulse duration, and lower beam spot size. An exposure which is therapeutic in one location may be ineffective in another, and may produce a hemorrhage in yet another.
Current clinical practice relies on visual assessment of pigmentation and vascularization and upon visual estimates of coloration changes that occur after a treatment pulse. However, such visual assessments are recognized by the art to be inadequate for reliable determination of the optimal laser application. Furthermore, there is recognition in the art of the need for a means of monitoring laser photocoagulation during treatment. As stated in an article entitled "Fundus Reflectometry: A Step Towards Optimization Of The Retina Photocoagulation" by R. Birngruber, V. -P. Gabel and F. Hillenkamp in Mod. Probl. Ophthal., vol. 18, 1977, pp. 383-390 at p. 383:
"Improvements in clinical retinal photocoagulation can be achieved by both the optimal adaptation of the instrument to the problem, and variation of the physical irradiation parameters . . . . It is obvious from the theory of heat conduction, that in the range of exposure times from 10.sup.-3 to 10 sec of interest here, the energy necessary for a given reaction in the irradiated area decreases markedly with shorter times. . . . It is understood though, that the possibility of manual control of the effect through visual observation of the coagulation site ceases for exposure times below about 1 sec.
Monitoring the time development of the retinal blanching during and after coagulation with suitable photodetectors should result in a more direct measure of the influence of the important parameters such as energy and exposure time on the retinal reaction. Such a technique could moreover eventually lead to a method for an automatic control of exposure times, even for very short times."
In searching for a means of monitoring the progress of laser photocoagulation during treatment, it has been recognized in the art that there is a connection between reflectivity of the irradiated tissues and the effects of the photocoagulation, see for example an article entitled "Time And Location Analysis Of Lesion Formation In Photocoagulation" by Oleg Pomerantzeff, Guang-Ji Wang, Michail Pankratov, and Julianne Schneider in Arch. Ophthalmol., Vol. 101, June, 1983, pp. 954-957. This has suggested the use of reflectometry, i.e. measurement of light backscattered from an illuminated spot on the retina, to monitor photocoagulation. The reflected light could come from the photocoagulation laser itself or from a secondary pilot laser.
In addition to attempts to monitor laser photocoagulation during treatment, there have been attempts in the art to pre-determine the appropriate laser dosages to apply for treatment of specific diseases. These attempts have used reflectometry to determine the laser parameters. Such a use of reflectometry is illustrated in an article entitled "A Method To Predetermine The Correct Photocoagulation Dosage" by Oleg Pomerantzeff, Guang-Ji Wang, Michail Pankratov, and Julianne Schneider in Arch. Ophthalmol., Vol. 101, June 1983, pp. 949-953, at p. 949:
"The most common goal of photocoagulation in the macular area is closing leakage from very small vessels and destroying new-formed vessels in the sub-retinal space. . . . Yellow and green light are recommended for treatment of the macula since the yellow pigment in the inner layers of this area absorbs very little of these colors. . . . The reaction of retinal tissue to the irradiation with a given power density varies according to the local concentrations of blood and melanin. Therefore, to avoid overtreatment and the risk of hemorrhage, it is desirable to know this relative concentration in the target tissues before treatment is applied, especially when red light is used. In this study, we suggest a possible method to measure this relative concentration.
Absorbance cannot be measured directly in a living eye but it can be measured indirectly by measuring reflectance. To do this we assume that the light that is neither absorbed by nor scattered back from the retinal or chloroidal layers reaches the sclera, which transmits only a negligible percentage, and is reflected from it. This reflected light is partly absorbed on its way back, and finally emerges from the retina into the vitreous. Therefore, if we measure the power applied to the retina and the power emerging back from the retina, the difference between the two is a measure of the absorbance in the retina and choroid. In photocoagulation it is also important to determine, if possible, the level within which most of the melanin is concentrated."
at p. 950:
"Since the reflection by the retinal structures is most diffuse, we are obviously not collecting all the light reflected from the retina. However, we may assume that the ratio of the collected to the reflected light remains the same, at least in the same eye."
and at p. 952:
"Not all the light emerging from the cornea is diffusely reflected. There are also some discrete specular reflections that may eventually fail into the entrance pupil of the measuring system, making the measurements unreliable. . . . The reflectance, and consequently the absorbance, depends not only on the retinal area and the selected wavelength, but also on the angle at which the particular structure is irradiated. Therefore the absorbance should be measured using the coagulating beam in its coagulating position."
In sum, fundus reflectometry, i.e. measurement of light that is backscattered from an illuminated spot on the retina, is used in the art to pre-determine laser photocoagulation dosages as well as to monitor laser photocoagulation during treatment. In theory, if the intensity of the incident radiation is known, the absorbance of the tissue can be calculated from the reflectance, and if the absorbance is known, the amount of energy absorbed from a treatment can be predicted. However, application of this theory involves a number of complications, such as: (1) wavelength dependence of the scattering, (2) angular dependence of the scattering, (3) the relation between the scattered light which leaves the pupil of the eye and is therefore accessible for measurement and the scattered light which is re-absorbed inside the eye and therefore cannot be directly measured. Furthermore, in prior art apparatus constructed to apply fundus reflectometry, reflection of the incident light from filters, lenses and other optical transmission components is quite strong, being at least a few percent of the incident radiation. This means that simple reflectance from a target cannot be easily measured when the same optical system is used to deliver the light to the target and to capture the reflected light. However, when two separate optical systems are used for simple reflectance measurement, it is difficult to insure that they are both aimed at precisely the same target spot.